Nickel-iron battery with high cycle life

ABSTRACT

The present invention provides one with a Ni—Fe battery exhibiting enhanced power characteristics. The battery uses a particular electrolyte. The resulting characteristics of specific power and power density are much improved over conventional Ni—Fe batteries. The electrolyte comprises sodium hydroxide, with lithium hydroxide and sodium sulfide. The iron anode comprises an iron active material and a polyvinyl alcohol binder.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims priority to U.S. Provisional Application Ser. No. 61/761,312, filed Feb. 6, 2013, and U.S. Provisional Application Ser. No. 61/927,291, filed Jan. 14, 2014, which applications are incorporated herein by reference in their entirety.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention is in the technical field of energy storage devices. More particularly, the present invention is in the technical field of rechargeable batteries using an iron anode and a nickel cathode.

2. State of the Art

Iron electrodes have been used in energy storage batteries and other devices for over one hundred years. Iron electrodes are often combined with a nickel base cathode to form a nickel-iron battery. The nickel-iron battery (Ni—Fe battery) is a rechargeable battery having a nickel (III) oxide-hydroxide cathode and an iron anode, with an electrolyte such as potassium hydroxide. The active materials are held in nickel-plated steel tubes or perforated pockets. It is a very robust battery which is tolerant of abuse, (overcharge, overdischarge, and short-circuiting) and can have a relatively long life. It is often used in backup situations where it can be continuously charged and can last for more than 20 years. Due to its low specific energy, poor charge retention, and high cost of manufacture, however, other types of rechargeable batteries have displaced the nickel-iron battery in most applications.

The ability of these batteries to survive frequent cycling is due to the low solubility of the reactants in the electrolyte. The formation of metallic iron during charge is slow because of the low solubility of the ferrous hydroxide. While the slow formation of iron crystals preserves the electrodes, it also limits the high rate performance. These cells charge slowly, and are only able to discharge slowly. Nickel-iron cells should not be charged from a constant voltage supply since they can be damaged by thermal runaway. The cell internal voltage drops as gassing begins, raising the temperature, which increases current drawn and so further increases gassing and temperature.

The industry, however, would be greatly served by a nickel-iron battery having improved performance. Such batteries, having even higher cycle life, particularly in combination with improved charge retention, specific power and power density, would be greatly welcome. The uses of nickel-iron batteries would thereby be increased.

SUMMARY OF THE INVENTION

The present invention provides one with a battery having an iron anode, e.g., a Ni—Fe battery, having improved performance characteristics. The battery uses a particular electrolyte. The resulting characteristics of cycle life, charge retention, specific power and power density are much improved over such iron anode batteries in the prior art.

Among other factors, it has been discovered that when using an iron anode in a battery, the use of a particular electrolyte enhances the performance characteristics of the battery significantly. The electrolyte is a sodium hydroxide based electrolyte. In one embodiment, a separator is used that is a non-treated polymeric separator, e.g., made from a polyolefin. The result is a battery of enhanced power, capacity and efficiency. The cycle life can be improved tenfold compared to the prior art.

BRIEF DESCRIPTION OF THE FIGURES OF THE DRAWING

FIG. 1 is a perspective view of a coated iron anode;

FIG. 2 is a side view and cross-section view of an iron electrode coated on both sides of the substrate;

FIG. 3 is a schematic of a battery in accordance with one embodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

For the purpose of the present invention and description, the following definitions will apply.

-   -   Capacity of a battery is measured in ampere hours (Ah).     -   Specific energy defines the battery capacity in weight, Watt         hours/kg (WH/kg). A battery can have a high specific energy but         poor specific power (load capacity), as is the case in alkaline         batteries. A battery may have a low specific energy but can         deliver high specific power, as is possible, e.g., with a         supercapacitor. Specific energy is often thought to be         synonymous with battery capacity and runtime.     -   Energy density, or volumetric energy density, is given in size,         Watt hours/liter (WH/L).     -   Specific power defines the battery capacity, or the amount of         current the battery can provide. Specific power is given in         Watts/kg (W/kg). Batteries for power tools, for example, often         exhibit high specific power but low capacity. Specific power         indicates internal resistance and the delivery of power.     -   Power density is the amount of power per unit volume. Power         density is given in Watts/liter (W/L).     -   C rate specifies charge and discharge currents. At 1 C, the         battery charges and discharges at a current that is par with the         marked Ah rating. At 0.5 C, the current is half, and at 0.1 C         the current is one tenth. For example, 1 C charges a battery in         about one hour; 0.5 C would take 2 hours and 0.1 C about 10         hours.     -   Watt hour efficiency is the energy discharged as a percentage of         energy charged.     -   Charge retention is the capacity measured after 28 days at 20°         C.     -   Cycle life of the battery is an important aspect, and is         measured at 80% DOD (depth of discharge), at 20° C., 1 C charge,         1 C discharge, to 70% capacity.

The invention comprises a battery with an iron anode and nickel cathode. The battery, in one embodiment comprises an iron electrode comprised of a single, coated conductive substrate, prepared by a simple coating process, which can be continuous. The substrate can be coated on one side, or on both sides.

The battery is prepared by conventional processing and construction with an iron anode and a cathode, i.e., a nickel cathode. The battery of the present invention, however, comprises a particular electrolyte. In one embodiment, the nickel-iron battery also comprises a particular separator, e.g., comprised of a polyolefin. In one embodiment, the iron electrode is comprised of a single, coated conductive substrate, as shown in FIGS. 1 and 2.

Turning to the figures of the drawing, FIG. 1 is a prospective view of a coated iron electrode. The substrate 1 is coated on each side with the coating 2 comprising the iron active material and binder. This is further shown in FIG. 2. The substrate 10 is coated on each side with the coating 11 of the iron active material and binder.

The electrolyte used is a sodium hydroxide based electrolyte, with the sodium hydroxide generally having a concentration of 5-7N in the electrolyte. In one embodiment, the electrolyte comprises sodium hydroxide, lithium hydroxide and sodium sulfide. For example, the sodium hydroxide concentration in the electrolyte is about 6N, the lithium hydroxide concentration in the electrolyte is about 1N, and the sodium sulfide concentration in the electrolyte is about 2 wt %. In using this electrolyte with an iron anode battery, it has been discovered that the life, capacity and power of the battery is much improved. It is believed that the use of the sodium hydroxide based electrolyte reduces the iron solubility in the electrolyte, which extends the battery life. The entire cell is also more stable and effective at high temperatures. The lithium hydroxide increases charge acceptance of the positive electrode, which increases capacity.

The presence of a metal sulfide has been discovered to be important for the effective deposit of sulfur on the iron anode. A battery with an iron anode seems to work better with sodium sulfide in the electrolyte, as the sulfide ends up in the iron anode as a performance enhancer after a few cycles. The sodium sulfide in essence is believed to increase the effective surface area of the iron, so one obtains more utilization of the iron. The capacity and power is therefore improved. In addition, the added sulfide is believed to form iron sulfides, two of the forms being FeS and Fe₂S₃, both of which are more electrically conductive than Fe(OH)₂ which normally forms on the iron surface. These conductive sites on the iron surface create a situation in which the oxidized layer that forms on the iron surface is thicker before true electrical passivation occurs allowing for more efficient use of the underlying iron active material. Various sulfide salts may be employed to obtain this desirable result. In one embodiment, the sulfide salt is sodium sulfide. Overall, it has been found that use of the present electrolyte improves cycle life, as well as the power and the capacity (charge retention) of a standard Ni—Fe battery.

While the use of metal sulfides such as sodium sulfide is described above, it will be understood that other sulfide compounds of suitable solubility may also be used. Examples of such sulfides include inorganic sulfides with sufficient solubility, but also organic sulfur compounds known to decompose in the electrolyte to inorganic sulfide.

It has also been found that the concentration of sulfide per se in the electrolyte can be important. In one embodiment, the amount of sulfide per se, i.e., the amount of sulfide itself, as measured as a percentage of the weight of electrolyte, is from 0.23% to 0.75%. In one embodiment, the amount of sulfide per se, measured as a percentage of the iron in the electrode, ranges from 0.23 wt % to 0.75 wt %.

The metal sulfide is preferably Na₂S. The sodium sulfide can be, for example, hydrated Na₂S. Hydrated sodium sulfide is about 60% Na₂S by weight, and this must be considered in calculating the amount of sulfide per se used in the electrolyte. In general, the amount of Na₂S used in the electrolyte ranges from 1-2 wt %, based on the weight of the electrolyte.

In one embodiment, the concentration of the NaOH in the electrolyte is in the range of from 6 to 7.5M. In one embodiment, the amount of LiOH in the electrolyte is in the range of from 0.5 to 2.0M, and most preferably about 1.0M. The combination of NaOH with LiOH and a sulfide is unique in its effective result.

It has also been discovered that using the electrolyte described above in combination with an iron electrode coated onto a single substrate significantly reduces the time required for activation of the cell or battery. In particular, use of this electrolyte in conjunction with an adhering type of iron electrode comprising iron active materials pasted onto a conductive substrate such as a metal foil or foam, results in a battery with improved performance over Ni—Fe batteries of conventional pocket plate design. Performance is further improved if such an adhering type of iron electrode contains sulfur or sulfide additives.

The battery separator that can be used in the present battery in combination with the electrolyte, is one that is iron-phobic. The separator can be etched for wettability, but this is merely optional when using the present battery separator. The battery separator is made of a polymer, with a generally smooth surface. The polymer can be any polymer which provides a non-polar surface, which is also generally very smooth. Examples of such polymers include polyolefins, such as polyethylene, and polytetrafluoroethylene (e.g., Teflon®). By using a separator which is more iron-phobic, the separator picks up iron at a slower rate. This results in a significant increase in the cycle life of the battery. Use of the e.g., polyolefin, separator in combination with sodium hydroxide electrolyte has been discovered to improve the capacity, power and, the efficiency, but most importantly, the cycle life of a standard Ni—Fe battery at least threefold.

FIG. 3 depicts a battery 20 with an iron anode 21. A nickel cathode 22 is also in the battery. The electrolyte 23 surrounds both the iron anode and cathode. The electrolyte is the sodium hydroxide based electrolyte described above, comprising sodium hydroxide, lithium hydroxide and sodium sulfide. The battery separator 24 is in one embodiment an iron-phobic battery separator having a non-polar surface. The battery separator can be made of any substance that provides such a non-polar surface. Polymers are good candidates as they provide smooth and non-polar surfaces. Suitable polymers include the polyolefins.

The battery can be made using conventional means and processes. However, the anode must be an iron anode, and an electrolyte comprising sodium hydroxide, lithium hydroxide and sodium sulfide is used. In one embodiment, both the sodium hydroxide based electrolyte and an iron-phobic battery separator are used in the battery. A great benefit of using the three component sodium hydroxide based electrolyte is that the battery can be sealed. A typical flooded design need not be used. Such a sealed battery is maintenance free as electrolyte need not be added periodically, as one would with a flooded design.

In one embodiment, the iron anode itself is different from the traditional pocket anode design. The anode is a single, coated conductive substrate, which can be coated on one side, or both sides. The anode can also be made by a simple coating process, which can be continuous.

The single substrate of the iron anode is used as a current conducting and collecting material that houses the active material (iron) of the electrode. In the traditional pocket design, the substrate encompasses the active material and holds the material. Two layers of substrate are therefore required per electrode. In the single substrate design, a single layer of substrate is used. This single layer acts as a carrier with coated material bonded to at least one side. In one embodiment, both sides of the substrate are coated. This substrate may be a thin conductive material such as a metal foil or sheet, metal foam, metal mesh, woven metal, or expanded metal. For example, a 0.060 inch, 80 ppi, nickel foam material has been used.

The coating mix for the iron anode is a combination of binder and active materials in an aqueous or organic solution. The mix can also contain other additives such as pore formers. Pore formers are often used to insure sufficient H₂ movement in the electrode. Without sufficient H₂ diffusion, the capacity of the battery will be adversely affected. The binder materials have properties that provide adhesion and bonding between the active material particles, both to themselves and to the substrate current carrier. The binder is generally resistant to degradation due to aging, temperature, and caustic environment. The binder can comprise polymers, alcohols, rubbers, and other materials, such as an advanced latex formulation that has been proven effective. A polyvinyl alcohol binder is used in one embodiment.

The active material for the mix formulation of the iron anode is selected from iron species that are generally less oxidative. Such materials include metal Fe and iron oxide materials. The iron oxide material will convert to iron metal when a charge is applied. A suitable iron oxide material includes Fe₃O₄. In addition, any other additives are generally required to be of a less oxidative nature, such as sulfur, antimony, selenium, and tellurium.

The coating method can be a continuous process that applies the active material mixture to the substrate by a method such as spraying, dip and wipe, extrusion, low pressure coating die, or surface transfer. A batch process can also be used, but a continuous process is advantageous regarding cost and processing. The coating mixture has to maintain a high consistency for weight and thickness and coating uniformity. This method is conducive to layering of various materials and providing layers of different properties such as porosities, densities and thicknesses. For example, the substrate can be coated with three layers. The first layer being of high density, second layer of medium density, and final layer of a lower density to create a density gradient which improves the flow of gases from the active material to the electrolyte, and provides better electrolyte contact and ionic diffusion with the active material throughout the structure of the electrode.

After coating, the electrode is dried to remove any residual liquid, i.e., aqueous or organic solvent. The drying methods will generally provide a continuous method for liquid removal from the coated active material which will enhance the adhesion and binding effects of the dry constituents without iron ignition. This drying method provides a uniform and stable active material coating with the substrate material. Two stages of drying can be used. For example, the first can be radiation for bulk drying, for cost and quality control, followed by convection drying to remove the remaining liquid. The radiation used can be any radiation, such as infrared, microwave or UV, and is very fast. However, the radiation creates a high temperature at the surface of the coated electrode. The high temperature is fine as long as water is still present to act as a heat sink. Therefore, the water is generally removed to about 10-20 wt % water. This can generally be determined using a control chart. Going below 10% water is dangerous, as the electrode becomes too dry and the high temperature can ignite the iron. Thus, using the convention drying to complete the removal of water/liquid is a preferred embodiment, once the amount of water remaining is in the 10-20 wt % range. In another embodiment, radiation can be used to complete the drying if the process is conducted in an inert atmosphere.

The compaction methods used can be accomplished by rolling mill, vertical pressing, and magnetic compaction of the active material to the desired thickness from 0.005 to 0.500 inches and porosities from 10% to 50%, for high quality and low cost continuous processing. In one embodiment, the porosity of the electrode is from 15-25% porosity. This compaction method can be used in conjunction with the layering method described above for providing material properties of density, thickness, porosity, and mechanical adhesion.

In addition, continuous in-line surface treatments can be applied continuously throughout any of the steps including coating, layering, and drying processes. The treatments can apply sulfur, polymer, metal spray, surface lament, etc.

The present batteries including the iron electrode can be used, for example, in a cellphone, thereby requiring an electrode with only a single side coated. However, both sides are preferably coated allowing the battery to be used in numerous additional applications.

The resulting battery shows improved performance characteristics, and in particular an improved cycle life, e.g., a cycle life of at least 10,000 cycles, when cycled @ 80% DOD, at 20° C., 70% capacity. Comparison to a standard Ni—Fe battery of flooded design, the present high cycle Ni—Fe battery has been found to exhibit the following characteristics:

Invention Conventional WH/kg (specific energy) 105 50 WH/L (energy density) 183 40 W/kg (specific power) 2,100 100 W/L (power density) 3,660 80 WH efficiency 95% 65% Charge Retention 95% 60% (Capacity @ 28 days 20° C.) Cycle Life (@80% DOD, 20° C., 10,000 1,000 1 C Charge, 1 C Discharge, to 70% Capacity) In the foregoing table, WH is Watt hours.

In the foregoing comparison, the invention Ni—Fe battery used an electrolyte comprised of sodium hydroxide (NaOH), lithium hydroxide (LiOH), and sodium sulfide (Na₂S). The separator used in the inventive Ni—Fe battery was a 0.010 inch thick polyolefin non-woven mesh. The electrolyte used in the conventional Ni—Fe battery was potassium hydroxide (KOH), and the battery separator was 0.060 inch thick polyvinyl chloride (PVC) windows. The results show a vast improvement in performance characteristics for the inventive Ni—Fe battery. The Ni—Fe battery, in addition to having a cycle life of at least 10,000 cycles, can also have a capacity at 28 day, 20° C., of at least 95%; a power density of at least 3,660 W/L and specific power of at least 2100 W/kg.

While the foregoing written description of the invention enables one of ordinary skill to make and use what is considered presently to be the best mode thereof, those of ordinary skill will understand and appreciate the existence of variations, combinations, and equivalents of the specific embodiment, method, and examples herein. The invention should therefore not be limited by the above described embodiment, method, and examples, but by all embodiments and methods within the scope and spirit of the invention. 

What is claimed is:
 1. A battery, comprising: a nickel cathode; an iron anode comprising an iron active material that comprises metal iron or an iron oxide material and a polyvinyl alcohol binder; and an electrolyte comprising sodium hydroxide, lithium hydroxide, and sodium sulfide, with the amount of sulfide in the electrolyte ranging from 0.23% to 0.75% based on the weight of the electrolyte; and with the battery exhibiting a cycle life of at least about 10,000 cycles.
 2. The battery of claim 1, further comprising a polyolefin battery separator.
 3. The battery of claim 1, which is a sealed battery.
 4. The battery of claim 1, wherein the iron anode is comprised of a single layer of a conductive substrate coated on at least one side with a coating comprising the iron active material and the polyvinyl alcohol binder.
 5. The battery of claim 4, wherein the substrate is coated on both sides.
 6. The battery of claim 1, further exhibiting a specific energy of at least about 105 watt hours/kg.
 7. The battery of claim 1, further exhibiting an energy density of at least about 183 watt hours/liter.
 8. The battery of claim 1, further exhibiting a specific power of at least about 2100 watts/kg.
 9. The battery of claim 1, further exhibiting a power density of at least about 3660 watts/liter.
 10. The battery of claim 1, further exhibiting a watt hour efficiency of at least about 95%.
 11. The battery of claim 1, further exhibiting a charge retention, measured as capacity at 28 days 20° C., of at least about 95%.
 12. The battery of claim 1, exhibiting a specific energy of at least about 105 watt hours/kg; an energy density of at least about 183 watt hours/liter; a watt hour efficiency of at least about 95%; and a charge retention of at least about 95%. 